Diversified motion using multiple global motion models

ABSTRACT

A method for encoding a current frame of a video includes jointly determining respective motion models for reference frames and encoding the current frame using the respective motion models. The reference frames are used for encoding the current frame. Jointly determining respective motion models for reference frames includes determining respective aggregated residuals for combinations of candidate motion models and selecting the combination of candidate motion models that corresponds to the smallest aggregated residual. The respective motion models correspond to the candidate motion models of the selected combination.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority to and the benefit of U.S. ProvisionalApplication Patent Ser. No. 62/587,025, filed Nov. 16, 2017, the entiredisclosure of which is hereby incorporated by reference.

BACKGROUND

Digital video streams may represent video using a sequence of frames orstill images. Digital video can be used for various applications,including, for example, video conferencing, high-definition videoentertainment, video advertisements, or sharing of user-generatedvideos. A digital video stream can contain a large amount of data andconsume a significant amount of computing or communication resources ofa computing device for processing, transmission, or storage of the videodata. Various approaches have been proposed to reduce the amount of datain video streams, including compression and other encoding techniques.

Encoding based on motion estimation and compensation may be performed bybreaking frames or images into blocks that are predicted based on one ormore prediction blocks of reference frames. Differences (i.e., residualerrors) between blocks and prediction blocks are compressed and encodedin a bitstream. A decoder uses the differences and the reference framesto reconstruct the frames or images.

SUMMARY

A method for encoding a current frame of a video includes jointlydetermining respective motion models for reference frames and encodingthe current frame using the respective motion models. The referenceframes are used for encoding the current frame. Jointly determiningrespective motion models for reference frames includes determiningrespective aggregated residuals for combinations of candidate motionmodels and selecting the combination of candidate motion models thatcorresponds to the smallest aggregated residual. The respective motionmodels correspond to the candidate motion models of the selectedcombination.

An apparatus for encoding a current frame of a video according toanother aspect of the disclosure includes a processor and anon-transitory storage medium. The processor is configured to executeinstructions stored in the non-transitory storage medium to jointlydetermine motion models for reference frames such that a respectivemotion model is determined for a respective reference frame, and encodethe current frame using the respective motion models.

A method for decoding a current frame includes decoding, from an encodedbitstream, respective motion models for reference frames of the currentframe and, in response to determining that a current block of thecurrent frame is encoded using at least one of the respective motionmodels, decoding the current block using the at least one of therespective motion models. The respective motion models are jointlydetermined by an encoder.

These and other aspects of the present disclosure are disclosed in thefollowing detailed description of the embodiments, the appended claims,and the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The description herein makes reference to the accompanying drawingsdescribed below, wherein like reference numerals refer to like partsthroughout the several views.

FIG. 1 is a schematic of a video encoding and decoding system.

FIG. 2 is a block diagram of an example of a computing device that canimplement a transmitting station or a receiving station.

FIG. 3 is a diagram of a video stream to be encoded and subsequentlydecoded.

FIG. 4 is a block diagram of an encoder according to implementations ofthis disclosure.

FIG. 5 is a block diagram of a decoder according to implementations ofthis disclosure.

FIG. 6 is a flowchart diagram of a process for encoding a current blockusing segmentation-based parameterized motion models according to animplementation of this disclosure.

FIG. 7 is a diagram of a frame segmentation according to implementationsof this disclosure.

FIG. 8 is an illustration of examples of motion within a video frameaccording to implementations of this disclosure.

FIGS. 9A-D are illustrations of examples of warping pixels of a block ofa video frame according to a parameterized motion model according toimplementations of this disclosure.

FIG. 10 is a flowchart diagram of a process for decoding a current blockusing segmentation-based parameterized motion models according to animplementation of this disclosure.

FIG. 11 is an example of global motion according to implementations ofthis disclosure.

FIG. 12 is an example of encoding a current frame using global motionaccording to implementations of this disclosure.

FIG. 13 is an example of diversified global motion according toimplementations of this disclosure.

FIG. 14 is a flowchart diagram of a process for encoding a current frameof video using diversified motion according to an implementation of thisdisclosure.

FIG. 15 is a flowchart diagram of a process for jointly determiningmotion models according to an implementation of this disclosure.

FIG. 16 is an example of combinations of motion models according toimplementations of this disclosure.

FIG. 17 is a flowchart diagram of a process for jointly determiningmotion models according to another implementation of this disclosure.

FIG. 18 is a flowchart diagram of a process for jointly determiningmotion models according to yet another implementation of thisdisclosure.

FIG. 19 is a flowchart diagram of a process for decoding a current frameaccording to one implementation of this disclosure.

DETAILED DESCRIPTION

As mentioned above, compression schemes related to coding video streamsmay include breaking images into blocks and generating a digital videooutput bitstream (i.e., an encoded bitstream) using one or moretechniques to limit the information included in the output bitstream. Areceived bitstream can be decoded to recreate the blocks and the sourceimages from the limited information. Encoding a video stream, or aportion thereof, such as a frame or a block, can include using temporalor spatial similarities in the video stream to improve codingefficiency. For example, a current block of a video stream may beencoded based on identifying a difference (residual) between thepreviously coded pixel values, or between a combination of previouslycoded pixel values, and those in the current block.

Encoding using spatial similarities can be known as intra prediction.Intra prediction attempts to predict the pixel values of a block of aframe of a video stream using pixels peripheral to the block; that is,using pixels that are in the same frame as the block but that areoutside the block.

Encoding using temporal similarities can be known as inter prediction.Inter prediction attempts to predict the pixel values of a block of acurrent frame using a possibly displaced block or blocks from one ormore reference frames. A reference frame is a frame (i.e., a picture)that appears earlier or later in time in the video stream than thecurrent frame. A reference frame that appears later in time than thecurrent frame may be received by a decoder before the current frame. Forexample, the compressed bitstream 420 of FIG. 5 can be organized suchthat the later-in-time reference frame is included before the currentframe. Inter prediction can be performed using a motion vector thatrepresents translational motion, that is, pixel shifts of a predictionblock in a reference frame in the x- and y-axes as compared to the blockbeing predicted. Some codecs use up to eight reference frames, which canbe stored in a frame buffer. The motion vector can refer to (i.e., use)one of the reference frames of the frame buffer.

Two predictor blocks can be combined to form a compound predictor for ablock or region of a video image. A compound predictor can be created bycombining two or more predictors determined using, for example, theaforementioned prediction methods (i.e., inter and/or intra prediction).For example, a compound predictor can be a combination of a firstpredictor and a second predictor, which can be two intra predictors(i.e., intra+intra), an intra predictor and an inter predictor (i.e.,intra+inter), or two inter predictors (i.e., inter+inter).

The video compression and decompression methods of motion compensationdescribed above (herein referred to as translational motion compensationor translational motion) assume purely translational motion betweenblocks. Translational motion compensation models are performed asrectangular transformations. Translational motion determined at theblock level is referred to herein as “regular motion compensation.”

However, not all motion within a block can be described and/orefficiently described using translational motion models with respect toa reference block of a reference frame. For example, some motion mayinclude scaling, shearing, or rotating motion, either alone or withtranslational motion. Such motion can be attributed, for example, tocamera motion and is applicable to all, or at least many, blocks of aframe. As such, the motion is “global” to a frame. As mentioned andfurther described below, the global motion can itself be a translationalmotion. As such, predicting blocks of a current frame using atranslational global motion model can result in better performance(e.g., improved compression) than using local translational motion atthe block level (i.e., regular motion compensation). In encoding blocksusing inter prediction, the global motion may be used to produce areference block. Alternatively, the translational motion vector(s) foundby motion searching can be used.

Global motion may be represented by a “parameterized motion model” or“motion model.” A single motion model for each reference frame may notaccurately predict all of the underlying motion of the frame. Forexample, a single motion model for a reference frame performs well withrespect to rate-distortion optimization for video with consistentmotion. However, a video frame may include two or more moving segmentscomprising a collection of blocks of the video frame. The segments maycomprise, for example, one or more foreground objects moving alongdifferent directions and a background that moves along yet anotherdirection. In particular, for example, video with strong parallax maynot obtain consistent gains from using the single motion model.

Implementations of this disclosure describe the use of multiple motionmodels per reference frame. For several reference frames, the currentvideo frame may be segmented with respect to the reference frame, andparameterized motion models may be identified for the segments. Each ofthe parameterized motion models associated with a segment corresponds toa motion model type. “Segment” as used in connection with global motion,as detailed in the description of FIGS. 6-19, is to be distinguishedfrom a “segment” as used with respect to FIG. 3 below. A “segment” inconnection with global motion is a collection of blocks of a currentframe that may or may not be contiguous.

In some situations, whether one or more motion models are identified forone or more reference frames, the reference frames may not individuallyproperly describe the global motion in a current frame. As such, it canbe advantageous to identify the global motions jointly rather thanindependently (i.e., on a per reference frame basis). For example, ifthe global motions associated with a first reference frame accuratelydescribe a first portion of the current frame (e.g., the background ofthe current frame), then the global motions associated with a secondreference frame and/or a third reference frame can be used to describe(i.e., estimate) the global motion of the other portions of the currentframe. That is, in determining the motion models of the second and/orthe third reference frames, the first portion of the current frame canbe ignored, and motion models that fit the other portions (e.g., theforeground of the current frame) can be derived. By jointly inferringglobal motion across at least some of the reference frames available forcoding a current frame, the error associated with a residual can bereduced. Jointly identified (e.g., inferred, calculated, computed, etc.)global motion models is referred to herein as a diversified motionmodel.

Further details of techniques for using segmentation-based parameterizedmotion models for encoding and decoding a current block of a videoframe, and further details for jointly inferring global motion (i.e.,diversified motion) across at least some of the reference framesavailable for coding a current frame, are described herein with initialreference to a system in which they can be implemented.

FIG. 1 is a schematic of a video encoding and decoding system 100. Atransmitting station 102 can be, for example, a computer having aninternal configuration of hardware such as that described in FIG. 2.However, other suitable implementations of the transmitting station 102are possible. For example, the processing of the transmitting station102 can be distributed among multiple devices.

A network 104 can connect the transmitting station 102 and a receivingstation 106 for encoding and decoding of the video stream. Specifically,the video stream can be encoded in the transmitting station 102, and theencoded video stream can be decoded in the receiving station 106. Thenetwork 104 can be, for example, the Internet. The network 104 can alsobe a local area network (LAN), wide area network (WAN), virtual privatenetwork (VPN), cellular telephone network, or any other means oftransferring the video stream from the transmitting station 102 to, inthis example, the receiving station 106.

The receiving station 106, in one example, can be a computer having aninternal configuration of hardware such as that described in FIG. 2.However, other suitable implementations of the receiving station 106 arepossible. For example, the processing of the receiving station 106 canbe distributed among multiple devices.

Other implementations of the video encoding and decoding system 100 arepossible. For example, an implementation can omit the network 104. Inanother implementation, a video stream can be encoded and then storedfor transmission at a later time to the receiving station 106 or anyother device having memory. In one implementation, the receiving station106 receives (e.g., via the network 104, a computer bus, and/or somecommunication pathway) the encoded video stream and stores the videostream for later decoding. In an example implementation, a real-timetransport protocol (RTP) is used for transmission of the encoded videoover the network 104. In another implementation, a transport protocolother than RTP may be used, for example, a Hypertext TransferProtocol-based (HTTP-based) video streaming protocol.

When used in a video conferencing system, for example, the transmittingstation 102 and/or the receiving station 106 may include the ability toboth encode and decode a video stream as described below. For example,the receiving station 106 could be a video conference participant whoreceives an encoded video bitstream from a video conference server(e.g., the transmitting station 102) to decode and view, and furtherencodes and transmits his or her own video bitstream to the videoconference server for decoding and viewing by other participants.

FIG. 2 is a block diagram of an example of a computing device 200 thatcan implement a transmitting station or a receiving station. Forexample, the computing device 200 can implement one or both of thetransmitting station 102 and the receiving station 106 of FIG. 1. Thecomputing device 200 can be in the form of a computing system includingmultiple computing devices, or in the form of one computing device, forexample, a mobile phone, a tablet computer, a laptop computer, anotebook computer, a desktop computer, and the like.

A CPU 202 in the computing device 200 can be a conventional centralprocessing unit. Alternatively, the CPU 202 can be any other type ofdevice, or multiple devices, now existing or hereafter developed,capable of manipulating or processing information. Although thedisclosed implementations can be practiced with one processor as shown(e.g., the CPU 202), advantages in speed and efficiency can be achievedby using more than one processor.

A memory 204 in the computing device 200 can be a read-only memory (ROM)device or a random-access memory (RAM) device in an implementation. Anyother suitable type of storage device can be used as the memory 204. Thememory 204 can include code and data 206 that are accessed by the CPU202 using a bus 212. The memory 204 can further include an operatingsystem 208 and application programs 210, the application programs 210including at least one program that permits the CPU 202 to perform themethods described herein. For example, the application programs 210 caninclude applications 1 through N, which further include a video codingapplication that performs the methods described herein. The computingdevice 200 can also include a secondary storage 214, which can, forexample, be a memory card used with a mobile computing device. Becausethe video communication sessions may contain a significant amount ofinformation, they can be stored in whole or in part in the secondarystorage 214 and loaded into the memory 204 as needed for processing.

The computing device 200 can also include one or more output devices,such as a display 218. The display 218 may be, in one example, atouch-sensitive display that combines a display with a touch-sensitiveelement that is operable to sense touch inputs. The display 218 can becoupled to the CPU 202 via the bus 212. Other output devices that permita user to program or otherwise use the computing device 200 can beprovided in addition to or as an alternative to the display 218. Whenthe output device is or includes a display, the display can beimplemented in various ways, including by a liquid crystal display(LCD), a cathode-ray tube (CRT) display, or a light emitting diode (LED)display, such as an organic LED (OLED) display.

The computing device 200 can also include or be in communication with animage-sensing device 220, for example, a camera, or any otherimage-sensing device, now existing or hereafter developed, that cansense an image such as the image of a user operating the computingdevice 200. The image-sensing device 220 can be positioned such that itis directed toward the user operating the computing device 200. In anexample, the position and optical axis of the image-sensing device 220can be configured such that the field of vision includes an area that isdirectly adjacent to the display 218 and from which the display 218 isvisible.

The computing device 200 can also include or be in communication with asound-sensing device 222, for example, a microphone, or any othersound-sensing device, now existing or hereafter developed, that cansense sounds near the computing device 200. The sound-sensing device 222can be positioned such that it is directed toward the user operating thecomputing device 200 and can be configured to receive sounds, forexample, speech or other utterances, made by the user while the useroperates the computing device 200.

Although FIG. 2 depicts the CPU 202 and the memory 204 of the computingdevice 200 as being integrated into one unit, other configurations canbe utilized. The operations of the CPU 202 can be distributed acrossmultiple machines (wherein individual machines can have one or moreprocessors) that can be coupled directly or across a local area or othernetwork. The memory 204 can be distributed across multiple machines,such as a network-based memory or memory in multiple machines performingthe operations of the computing device 200. Although depicted here asone bus, the bus 212 of the computing device 200 can be composed ofmultiple buses. Further, the secondary storage 214 can be directlycoupled to the other components of the computing device 200 or can beaccessed via a network and can comprise an integrated unit, such as amemory card, or multiple units, such as multiple memory cards. Thecomputing device 200 can thus be implemented in a wide variety ofconfigurations.

FIG. 3 is a diagram of an example of a video stream 300 to be encodedand subsequently decoded. The video stream 300 includes a video sequence302. At the next level, the video sequence 302 includes a number ofadjacent frames 304. While three frames are depicted as the adjacentframes 304, the video sequence 302 can include any number of adjacentframes 304. The adjacent frames 304 can then be further subdivided intoindividual frames, for example, a frame 306. At the next level, theframe 306 can be divided into a series of planes or segments 308. Thesegments 308 can be subsets of frames that permit parallel processing,for example. The segments 308 can also be subsets of frames that canseparate the video data into separate colors. For example, a frame 306of color video data can include a luminance plane and two chrominanceplanes. The segments 308 may be sampled at different resolutions.

Whether or not the frame 306 is divided into segments 308, the frame 306may be further subdivided into blocks 310, which can contain datacorresponding to, for example, 16×16 pixels in the frame 306. The blocks310 can also be arranged to include data from one or more segments 308of pixel data. The blocks 310 can also be of any other suitable size,such as 4×4 pixels, 8×8 pixels, 16×8 pixels, 8×16 pixels, 64×64 pixels,128×128 pixels, or larger. Unless otherwise noted, the terms “block” and“macroblock” are used interchangeably herein.

FIG. 4 is a block diagram of an encoder 400 according to implementationsof this disclosure. The encoder 400 can be implemented, as describedabove, in the transmitting station 102, such as by providing a computersoftware program stored in memory, for example, the memory 204. Thecomputer software program can include machine-readable instructionsthat, when executed by a processor such as the CPU 202, cause thetransmitting station 102 to encode video data in the manner described inFIG. 4. The encoder 400 can also be implemented as specialized hardwareincluded in, for example, the transmitting station 102. In oneparticularly desirable implementation, the encoder 400 is a hardwareencoder.

The encoder 400 has the following stages to perform the variousfunctions in a forward path (shown by the solid connection lines) toproduce an encoded or compressed bitstream 420 using the video stream300 as input: an intra/inter prediction stage 402, a transform stage404, a quantization stage 406, and an entropy encoding stage 408. Theencoder 400 may also include a reconstruction path (shown by the dottedconnection lines) to reconstruct a frame for encoding of future blocks.In FIG. 4, the encoder 400 has the following stages to perform thevarious functions in the reconstruction path: a dequantization stage410, an inverse transform stage 412, a reconstruction stage 414, and aloop filtering stage 416. Other structural variations of the encoder 400can be used to encode the video stream 300.

When the video stream 300 is presented for encoding, respective adjacentframes 304, such as the frame 306, can be processed in units of blocks.At the intra/inter prediction stage 402, respective blocks can beencoded using intra-frame prediction (also called intra-prediction) orinter-frame prediction (also called inter-prediction). In any case, aprediction block can be formed. In the case of intra-prediction, aprediction block may be formed from samples in the current frame thathave been previously encoded and reconstructed. In the case ofinter-prediction, a prediction block may be formed from samples in oneor more previously constructed reference frames. Implementations forforming a prediction block are discussed below with respect to FIGS. 6,7, and 8, for example, using a parameterized motion model identified forencoding a current block of a video frame.

Next, still referring to FIG. 4, the prediction block can be subtractedfrom the current block at the intra/inter prediction stage 402 toproduce a residual block (also called a residual). The transform stage404 transforms the residual into transform coefficients in, for example,the frequency domain using block-based transforms. The quantizationstage 406 converts the transform coefficients into discrete quantumvalues, which are referred to as quantized transform coefficients, usinga quantizer value or a quantization level. For example, the transformcoefficients may be divided by the quantizer value and truncated. Thequantized transform coefficients are then entropy encoded by the entropyencoding stage 408. The entropy-encoded coefficients, together withother information used to decode the block (which may include, forexample, the type of prediction used, transform type, motion vectors,and quantizer value), are then output to the compressed bitstream 420.The compressed bitstream 420 can be formatted using various techniques,such as variable length coding (VLC) or arithmetic coding. Thecompressed bitstream 420 can also be referred to as an encoded videostream or encoded video bitstream, and the terms will be usedinterchangeably herein.

The reconstruction path in FIG. 4 (shown by the dotted connection lines)can be used to ensure that the encoder 400 and a decoder 500 (describedbelow) use the same reference frames to decode the compressed bitstream420. The reconstruction path performs functions that are similar tofunctions that take place during the decoding process (described below),including dequantizing the quantized transform coefficients at thedequantization stage 410 and inverse transforming the dequantizedtransform coefficients at the inverse transform stage 412 to produce aderivative residual block (also called a derivative residual). At thereconstruction stage 414, the prediction block that was predicted at theintra/inter prediction stage 402 can be added to the derivative residualto create a reconstructed block. The loop filtering stage 416 can beapplied to the reconstructed block to reduce distortion such as blockingartifacts.

Other variations of the encoder 400 can be used to encode the compressedbitstream 420. For example, a non-transform based encoder can quantizethe residual signal directly without the transform stage 404 for certainblocks or frames. In another implementation, an encoder can have thequantization stage 406 and the dequantization stage 410 combined in acommon stage.

FIG. 5 is a block diagram of a decoder 500 according to implementationsof this disclosure. The decoder 500 can be implemented in the receivingstation 106, for example, by providing a computer software programstored in the memory 204. The computer software program can includemachine-readable instructions that, when executed by a processor such asthe CPU 202, cause the receiving station 106 to decode video data in themanner described in FIG. 5. The decoder 500 can also be implemented inhardware included in, for example, the transmitting station 102 or thereceiving station 106.

The decoder 500, similar to the reconstruction path of the encoder 400discussed above, includes in one example the following stages to performvarious functions to produce an output video stream 516 from thecompressed bitstream 420: an entropy decoding stage 502, adequantization stage 504, an inverse transform stage 506, an intra/interprediction stage 508, a reconstruction stage 510, a loop filtering stage512, and a post filtering stage 514. Other structural variations of thedecoder 500 can be used to decode the compressed bitstream 420.

When the compressed bitstream 420 is presented for decoding, the dataelements within the compressed bitstream 420 can be decoded by theentropy decoding stage 502 to produce a set of quantized transformcoefficients. The dequantization stage 504 dequantizes the quantizedtransform coefficients (e.g., by multiplying the quantized transformcoefficients by the quantizer value), and the inverse transform stage506 inverse transforms the dequantized transform coefficients to producea derivative residual that can be identical to that created by theinverse transform stage 412 in the encoder 400. Using header informationdecoded from the compressed bitstream 420, the decoder 500 can use theintra/inter prediction stage 508 to create the same prediction block aswas created in the encoder 400, for example, at the intra/interprediction stage 402. At the reconstruction stage 510, the predictionblock can be added to the derivative residual to create a reconstructedblock. The loop filtering stage 512 can be applied to the reconstructedblock to reduce blocking artifacts.

Other filtering can be applied to the reconstructed block. In anexample, the post filtering stage 514 can include a deblocking filterthat is applied to the reconstructed block to reduce blockingdistortion, and the result is output as the output video stream 516. Theoutput video stream 516 can also be referred to as a decoded videostream, and the terms will be used interchangeably herein. Othervariations of the decoder 500 can be used to decode the compressedbitstream 420. For example, the decoder 500 can produce the output videostream 516 without the post filtering stage 514.

FIG. 6 is a flowchart diagram of a process 600 for encoding a currentblock using segmentation-based parameterized motion models according toan implementation of this disclosure. The process 600 can be implementedin an encoder such as the encoder 400 of FIG. 4.

The process 600 can be implemented, for example, as a software programthat can be executed by a computing device, such as the transmittingstation 102. The software program can include machine-readableinstructions (e.g., executable instructions) that can be stored in amemory, such as the memory 204 or the secondary storage 214, and thatcan be executed by a processor, such as the CPU 202, to cause thecomputing device to perform the process 600. In at least someimplementations, the process 600 can be performed in whole or in part bythe intra/inter prediction stage 402 of the encoder 400 of FIG. 4.

The process 600 can be implemented using specialized hardware orfirmware. Some computing devices can have multiple memories, multipleprocessors, or both. The steps or operations of the process 600 can bedistributed using different processors, memories, or both. Use of theterms “processor” or “memory” in the singular encompasses computingdevices that have one processor or one memory as well as devices thathave multiple processors or multiple memories that can be used in theperformance of some or all of the recited steps or operations.

The process 600 is described with reference to FIG. 7. FIG. 7 is adiagram 700 of a frame segmentation according to implementations of thisdisclosure. FIG. 7 includes a current frame 701. Blocks of the currentframe 701 can be encoded using reference frames, such as a referenceframe 704 and a reference frame 706, of a frame buffer 702. The currentframe 701 includes the head and shoulders of a person 720 and otherbackground objects.

At 602, the process 600 segments the video frame with respect to areference frame, resulting in a segmentation. The process 600 cansegment the video frame with respect to more than one reference frame.The segmentation can include one or more segments. The segmentationincludes a segment containing the current block and a parameterizedmotion model for the segment. FIG. 7 depicts three segments: a segment722 depicted by a group of shaded blocks, a segment 718 depicted byanother group of differently shaded blocks, and a segment 714, which isa group that includes the rest of the blocks of the frame constitutingthe background of the frame. The segment 722 includes the current block716.

For each of at least some of the reference frames of the frame buffer702, the process 600 can segment the current frame 701. The process 600can use an image segmentation technique that leverages the motion ofobjects between a reference frame and the current frame. A parameterizedmotion model is then associated with each segment, as is furtherdescribed with respect to FIG. 8.

Image segmentation can be performed using interest points to generatethe parameterized motion models. For example, the process 600 candetermine first interest points in a reference frame, such as thereference frame 704, and second interest points in the current frame701. The Features from Accelerated Segment Test (FAST) algorithm can beused to determine the first interest points and the second interestpoints. The first interest points and the second interest points arethen matched. The process 600 can use the matched interest points todetermine a parameterized motion model for the matching interest points.

The process 600 can use the Random Sample Consensus (RANSAC) method tofit a model (i.e., a parameterized motion model) to the matched points.RANSAC is an iterative algorithm that can be used to estimate modelparameters (i.e., the parameters of the parameterized motion model) fromdata that contain inliers and outliers. Inliers are the data points(i.e., pixels) of the current frame that fit the parameterized motionmodel. The process 600 can determine a segment based on the inliers.That is, the process 600 can include the inliers in one segment. Thesegment (referred to as a foreground segment) based on the inliers maycorrespond to motion in the current frame corresponding to foregroundobjects. However, that need not be the case. That is, the foregroundsegment may include background objects or blocks. The foreground segmentmay not include all foreground objects or blocks.

Outliers are the data points (i.e., pixels) of the current frame that donot fit the parameterized motion model. The process 600 can determine asecond segment based on the outliers. The segment (referred to as abackground segment) based on the outliers may correspond to relativelystatic background objects of the current frame. However, that need notbe the case. Alternatively, instead of determining a second segmentbased on the outliers, the process 600 can use the outliers to determineadditional segments. For example, the process 600 can recursively applythe same process as described above to determine additional segments.For example, by applying the process as described above to the currentframe 701 and using the reference frame 704, the process 600 determinesthe three segments 714, 718, and 722. The two segments 718 and 722 maybe identified for the person 720 in a case where, for example, theshoulders of the person 720 are moving, with respect to a referenceframe, in one direction while the head is moving another direction.

The process 600 can determine a parameterized motion model (for example,using RANSAC) based on a motion model type. For example, the RANSACalgorithm can determine a parameterized motion model based on a motionmodel type provided by the process 600. Different motion model types canbe available. Available motion model types include, in increasingcomplexity, a translational motion model type, a similarity motion modeltype, an affine motion model type, and a homographic motion model type.Additional or fewer motion model types may be available. Some of themotion model types are explained further with respect to FIGS. 9A-9D.

In some situations, the parameterized motion model determined by theRANSAC method may contain more parameters than are necessary to providea good approximation (e.g., with respect to an error metric) of theglobal motion for a segment. For example, requesting an affine modelfrom RANSAC may return a six-parameter model (as described with respectto FIGS. 9A-9D), even though a four-parameter model is sufficient toprovide a good approximation of the segment. As such, the process 600can iteratively evaluate the available model types starting from a leastcomplex model type (e.g., the translational motion model type) to a mostcomplex model (e.g., the homographic motion model type). If a lowercomplexity model is determined to produce an error metric within apredefined threshold, then the parameterized motion model correspondingto the lower complexity model is determined to be the parameterizedmotion model of the segment.

In an implementation, an error advantage associated with a model typecan be used as the error metric. The error advantage E can be defined as

E=Σa|c _(xy) −w _(xy)|^(0.6)

In the equation above, a is a weight value, c_(xy) is the pixel at (x,y) in the current frame, and w_(xy) is the pixel at (x, y) in the warpedframe, as described below. If a model type produces an error advantage Ebelow a predefined threshold, then the parameterized motion modelcorresponding to the model type is associated with the segment. If nomodel type produces an error advantage E below the predefined threshold,then the translational motion model type can be assumed for the segment.

In an implementation, the process 600 does not evaluate the homographicmotion model type; rather, the process 600 stops at the similaritymotion model type. This is so in order to reduce decoder complexity.

The process 600 can segment the current frame with respect to (or basedon) each of the reference frames of the frame buffer 702. In FIG. 7, theframe buffer 702 includes eight (8) reference frames. Assuming that theprocess 600 determines two (2) segments (i.e., a foreground segment anda background segment) per reference frame, the segmentation results in atotal of 16 segments. Each of the 16 segments corresponds to arespective parameterized motion model, resulting in 16 parameterizedmotion models.

As will be explained further with respect to FIG. 8, a prediction blockfor a current block is determined based on the available segments thatcontain the current block in the reference frames. As such, if anencoder, such as the encoder 400 of FIG. 4, determines a predictionblock using the 16 parameterized motion models, then a decoder, such asthe decoder 500 of FIG. 5, also uses the 16 parameterized motion modelsto reconstruct the current block. As such, the 16 parameterized motionmodels are encoded in an encoded bitstream, such as the bitstream 420generated by the encoder 400 and received by the decoder 500.

Encoding, in the encoded bitstream, the parameters of, for example, 16parameterized motion models may outweigh the prediction gains ofsegmentation-based parameterized motion models. As such, the process 600can determine a subset of the reference frames of the frame buffer thatresults in the best fit for a specific segment. For a segment, a numberof reference frames (e.g., three frames) are selected, and theparameterized motion models with respect to these frames are determinedand encoded in the encoded bitstream. For example, the process 600 candetermine, for a segment, the parameterized motion models based on thegolden reference frame, alternative reference frame, and the lastreference frame of the frame buffer. The golden reference frame can be areference frame available as a forward prediction frame for encoding acurrent frame. The last reference frame can be available as a forwardprediction frame for encoding the current frame. The alternativereference frame can be available as a backward reference frame forencoding the current frame.

Encoding a parameterized motion model can mean encoding the parametersof the parameterized motion model in the header of the current framebeing encoded. Encoding the parameterized motion model can includeencoding the motion model type corresponding to the parameterized motionmodel.

In the case where the motion model type is encoded, a decoder, such asthe decoder 500 of FIG. 5, decodes the motion model type and determinesthe parameters of the parameterized motion model of the motion modeltype in a way similar to that of the encoder. In order to limit decodercomplexity, the encoder can encode a motion model type that is lesscomplex than the most complex motion model type. That is, for example,the encoder can determine a parameterized motion model for a segmentusing a motion model type no more complex than the similarity motionmodel type.

Referring again to FIG. 7, the foreground segment 722 is obtained fromthe reference frame 704 (as indicated by line 708). The backgroundsegment 714 is obtained from the reference frame 706 (as indicated byline 710). That is, each segment can be obtained from a differentreference frame. However, this is not necessary. Some of the segmentscan be obtained from the same reference frames. For example, and asillustrated in FIG. 7, the foreground segment 722 and the backgroundsegment 714 can be obtained from the same reference frame 706 (asindicated by the line 712 and the line 710, respectively). The segment718, while not specifically indicated in FIG. 7, can also be obtainedfrom any of the reference frames of the frame buffer 702.

FIG. 8 is an illustration of examples of motion within a video frame 800according to implementations of this disclosure. While not specificallyindicated, it should be understood that the end points of the motiondirections (e.g., motion 808) of FIG. 8 refer to pixel positions withinreference frames. This is so because motion is described with respect toanother frame, such as a reference frame. The illustrated end points maynot be end points in the same reference frame. One or more of blocks 802within the video frame 800 can include warped motion. Warped motion ismotion that might not be accurately predicted using motion vectorsdetermined via translational motion compensation (e.g., regular motioncompensation as described above). For example, the motion within one ofthe blocks 802 might scale, rotate, or otherwise move in a not entirelylinear manner in any number of different directions. Alternatively, themotion within one of the blocks 802 may be translational motion that ismore efficiently described using global translational motion. As such, aprediction block used to encode or decode one of the blocks 802 can beformed, or generated, using a parameterized motion model.

Motion within the video frame 800 may include a global motion. A videoframe can contain more than one global motion. Blocks exhibiting thesame global motion can be grouped into a segment. The segments may ormay include contiguous and/or non-contiguous blocks.

The video frame 800 includes motion at 804, 806, and 808. The motion at804, 806, and 808 demonstrate, respectively, motion of pixels of theblocks 802A, 802B, and 802C. For example, the motion shown at 804 is atranslational motion of the pixels of the block 802A. The motion shownat 806 is a rotational motion of the pixels of the block 802B. Themotion shown at 808 is a warped motion of the pixels of the block 802C.The group of blocks of the video frame 800 exhibiting the same globalmotion can be grouped into one segment. For example, the motion 810 ofthe block 802D is the same as the motion of the block 802A. As such, theblocks 802A and 802D can be included in a same segment. More than oneglobal motion can be associated with the frame. Each global motion canbe associated with a segment of the frame. The blocks 802A-802D areillustrated, for simplicity, as including four pixels. However, asdescribed above, the blocks can include a larger number of pixels.

In addition to the global motions, the video frame 800 may have localmotion within a portion of the video frame 800. For example, a localmotion is shown at 812. Local motion within the video frame 800 may becontained within one block or within a number of adjacent ornon-adjacent blocks. The video frame 800 may include multiple differentlocal motions.

A frame header 814 of the video frame 800 includes references toreference frames available for encoding or decoding the blocks802A-802D. The references to the reference frames in the frame header814 can be for a parameterized motion model associated with thosereference frames. A parameterized motion model corresponds to a motionmodel type (described later with respect to FIGS. 9A-D) and indicateshow pixels of blocks (e.g., the blocks 802A-802D) of the video frame 800can be warped to generate prediction blocks usable for encoding ordecoding the blocks. The frame header 814 can include one or moreparameterized motion models, each corresponding to a segment of thevideo frame 800.

For example, the parameterized motion model 816 corresponds to a firstmotion model of a first segment associated with a first reference frame.The parameterized motion model 818 corresponds to a second motion modelof a second segment associated with the first reference frame. Theparameterized motion model 820 corresponds to a first motion model of afirst segment associated with a second reference frame. Theparameterized motion model 822 corresponds to a second motion model of asecond segment associated with the second reference frame. Theparameterized motion model 824 corresponds to a third motion model of athird segment associated with the second reference frame. Theparameterized motion model 826 corresponds to a first motion model of afirst segment associated with a third reference frame. The parameterizedmotion model 828 corresponds to a second motion model of a secondsegment associated with the third reference frame.

The parameterized motion models associated with a reference frame maycorrespond to one or more motion model types. For example, theparameterized motion model 816 and the parameterized motion model 818may respectively correspond to a homographic motion model and an affinemotion model for the first reference frame. In some implementations,each reference frame can be associated with multiple parameterizedmotion models of a single motion model type. For example, theparameterized motion model 816 and the parameterized motion model 818may both correspond to different homographic motion models. However, insome implementations, a reference frame may be limited to one motionmodel for each motion model type. Further, in some implementations, areference frame may be limited to a single motion model total. In such acase, that motion model may be replaced in certain situations, such aswhere a new motion model results in a lower prediction error.

A parameterized motion model may indicate a global motion withinmultiple frames of a video sequence. As such, the parameterized motionmodels encoded within the frame header 814 may be used to generateprediction blocks for multiple blocks in multiple frames of a videosequence. The reference frames associated with the parameterized motionmodels in the frame header 814 may be selected from a reference framebuffer, such as by using bits encoded to the frame header 814. Forexample, the bits encoded to the frame header 814 may point to virtualindex locations of the reference frames within the reference framebuffer.

FIGS. 9A-D are illustrations of examples of warping pixels of a block ofa video frame according to a parameterized motion model according toimplementations of this disclosure. A parameterized motion model used towarp pixels of a block of a frame can correspond to a motion model type.The motion model type that corresponds to a parameterized motion modelmay be a homographic motion model type, an affine motion model type, asimilarity motion model type, or a translational motion model type. Theparameterized motion model to use can be indicated by data associatedwith reference frames, such as within frame headers of an encodedbitstream.

FIGS. 9A-D depict different motion model types used to project pixels ofa block to a warped patch within a reference frame. The warped patch canbe used to generate a prediction block for encoding or decoding thatblock. A parameterized motion model indicates how the pixels of a blockare to be scaled, rotated, or otherwise moved when projected into thereference frame. Data indicative of pixel projections can be used toidentify parameterized motion models corresponding to a respectivemotion model. The number and function of the parameters of aparameterized motion model depend upon the specific projection used.

In FIG. 9A, pixels of a block 902A are projected to a warped patch 904Aof a frame 900A using a homographic motion model. A homographic motionmodel uses eight parameters to project the pixels of the block 902A tothe warped patch 904A. A homographic motion is not bound by a lineartransformation between the coordinates of two spaces. As such, the eightparameters that define a homographic motion model can be used to projectpixels of the block 902A to a quadrilateral patch (e.g., the warpedpatch 904A) within the frame 900A. Homographic motion models thussupport translation, rotation, scaling, changes in aspect ratio,shearing, and other non-parallelogram warping. A homographic motionbetween two spaces is defined as follows:

${x = \frac{{a*X} + {b*Y} + c}{{g*X} + {h*Y} + 1}};{{{and}\mspace{14mu} y} = {\frac{{d*X} + {e*Y} + f}{{g*X} + {h*Y} + 1}.}}$

In these equations, (x, y) and (X, Y) are coordinates of two spaces,namely, a projected position of a pixel within the frame 900A and anoriginal position of a pixel within the block 902A, respectively.Further, a, b, c, d, e, f, g, and h are the homographic parameters andare real numbers representing a relationship between positions ofrespective pixels within the frame 900A and the block 902A. Of theseparameters, a represents a fixed scale factor along the x-axis with thescale of the y-axis remaining unchanged, b represents a scale factoralong the x-axis proportional to the y-distance to a center point of theblock, c represents a translation along the x-axis, d represents a scalefactor along the y-axis proportional to the x-distance to the centerpoint of the block, e represents a fixed scale factor along the y-axiswith the scale of the x-axis remaining unchanged, f represents atranslation along the y-axis, g represents a proportional scale offactors of the x- and y-axes according to a function of the x-axis, andh represents a proportional scale of factors of the x- and y-axesaccording to a function of the y-axis.

In FIG. 9B, pixels of a block 902B are projected to a warped patch 904Bof a frame 900B using an affine motion model. An affine motion modeluses six parameters to project the pixels of the block 902B to thewarped patch 904B. An affine motion is a linear transformation betweenthe coordinates of two spaces defined by the six parameters. As such,the six parameters that define an affine motion model can be used toproject pixels of the block 902B to a parallelogram patch (e.g., thewarped patch 904B) within the frame 900B. Affine motion models thussupport translation, rotation, scale, changes in aspect ratio, andshearing. The affine projection between two spaces is defined asfollows:

x=a*X+b*Y+c; and y=d*X+e*Y+f.

In these equations, (x, y) and (X, Y) are coordinates of two spaces,namely, a projected position of a pixel within the frame 900B and anoriginal position of a pixel within the block 902B, respectively. Also,a, b, c, d, e, and f are affine parameters and are real numbersrepresenting a relationship between positions of respective pixelswithin the frame 900B and the block 902B. Of these, a and d representrotational or scaling factors along the x-axis, b and e representrotational or scaling factors along the y-axis, and c and f respectivelyrepresent translation along the x- and y-axes.

In FIG. 9C, pixels of a block 902C are projected to a warped patch 904Cof a frame 900C using a similarity motion model. A similarity motionmodel uses four parameters to project the pixels of the block 902C tothe warped patch 904C. A similarity motion is a linear transformationbetween the coordinates of two spaces defined by the four parameters.For example, the four parameters can be a translation along the x-axis,a translation along the y-axis, a rotation value, and a zoom value. Assuch, the four parameters that define a similarity motion model can beused to project pixels of the block 902C to a square patch (e.g., thewarped patch 904C) within the frame 900C. Similarity motion models thussupport square-to-square transformation with rotation and zoom.

In FIG. 9D, pixels of a block 902D are projected to a warped patch 904Dof a frame 900D using a translational motion model. A translationalmotion model uses two parameters to project the pixels of the block 902Dto the warped patch 904D. A translational motion is a lineartransformation between the coordinates of two spaces defined by the twoparameters. For example, the two parameters can be a translation alongthe x-axis and a translation along the y-axis. As such, the twoparameters that define a translational motion model can be used toproject pixels of the block 902D to a square patch (e.g., the warpedpatch 904D) within the frame 900D.

Returning again to FIG. 6, at 604, the process 600 determines a firstmotion vector for the current block based on the segmentation. Asdescribed above, the video frame can be partitioned with respect to atleast some of the reference frames into segments. As such, the currentblock can be part of many segments, each corresponding to a referenceframe. For at least some of the segments that the current frame belongsto, the process 600 determines a respective motion vector.

A motion vector is generated between the current block and a referenceframe selected based on the parameterized motion model associated withthe segment of the current block as described with respect to FIGS. 7,8, and 9. As such, the motion vector between the current block and areference frame selected based on the parameterized motion model can bea reference to the parameterized motion model. That is, the motionvector indicates the reference frame and the parameterized motion model.

The motion vector can be generated by warping pixels of the currentblock to a warped patch within the reference frame according to theselected parameterized motion model. For example, the pixels of thecurrent block are projected to the warped patch within the referenceframe. The shape and size of the warped patch to which the pixels of thecurrent block are projected depends upon the motion model associatedwith the selected parameterized motion model. The warped patch can be arectangular patch or a non-rectangular patch. For example, if theparameterized motion model is of a translational motion model type, thewarped patch is a rectangular block that is the same size as the currentblock. In another example, if the parameterized motion model is of ahomographic motion model type, the warped patch may be any quadrilateraland of any size. The position of the warped patch also depends upon themotion model. For example, the parameters of the parameterized motionmodel indicate an x-axis and/or y-axis translation for the warped patch.The parameters of the parameterized motion model may further indicate arotation, zoom, or other motional change for the warped patch.

The warped patch can then be unwarped using the motion vector to returnthe current block to generate a prediction block. The prediction blockcan have a rectangular geometry for predicting the current block. Forexample, unwarping the projected pixels of the warped patch afterrespective pixels are projected to the warped patch of the referenceframe can include projecting the warped patch to a rectangular blockusing the generated motion vector. The pixel position coordinates of thewarped patch of the reference frame can be projected to the rectangularblock based on respective coordinate translations to the rectangularblock. The resulting rectangular block can be used to generate theprediction block.

At 606, the process 600 determines a second motion vector for thecurrent block using translational motion compensation at the block level(i.e., regular motion compensation). That is, the process 600 candetermine the second motion vector using inter prediction as describedabove.

At 608, the process 600 encodes, for the current block, the one of thefirst motion vector and the second motion vector corresponding to asmaller error. The smaller error can be the error corresponding to thebest rate-distortion value. A rate-distortion value refers to a ratiothat balances an amount of distortion (i.e., loss in video quality) withrate (i.e., the number of bits) used for encoding. For each of themotion vectors determined at 604 and 606, the process 600 can determinethe motion vector corresponding to the best rate-distortion value.

The process 600 can encode, in the encoded bitstream, the selectedmotion vector. In the case where the selected motion vector is asegmentation-based motion vector (i.e., a motion vector determined at604), the process 600 can encode the parameters of the parameterizedmotion model used to determine the motion vector. Alternatively, theprocess 600 can encode the motion model type corresponding to theparameterized motion model.

FIG. 10 is a flowchart diagram of a process 1000 for decoding a currentblock using segmentation-based parameterized motion models according toan implementation of this disclosure. The process 1000 receives anencoded bitstream, such as the compressed bitstream 420 of FIG. 5. Theprocess 1000 may be performed by a decoder. For example, the process1000 can be performed in whole or in part by the intra/inter-predictionstage 508 of the decoder 500. The process 1000 can be performed in wholeor in part during the reconstruction path (shown by the dottedconnection lines) of the encoder 400 of FIG. 4. Implementations of theprocess 1000 can be performed by storing instructions in a memory, suchas the memory 204 of the receiving station 106 or the transmittingstation 102, to be executed by a processor, such as the CPU 202, forexample.

The process 1000 can be implemented using specialized hardware orfirmware. Some computing devices can have multiple memories, multipleprocessors, or both. The steps or operations of the process 1000 can bedistributed using different processors, memories, or both. Forsimplicity of explanation, the process 1000 is depicted and described asa series of steps or operations. However, the teachings in accordancewith this disclosure can occur in various orders and/or concurrently.Additionally, steps or operations in accordance with this disclosure mayoccur with other steps or operations not presented and described herein.Furthermore, not all illustrated steps or operations may be used toimplement a method in accordance with the disclosed subject matter.

At 1002, the process 1000 identifies a parameterized motion modelcorresponding to a motion model type. The parameterized motion model canbe identified based on information encoded in a header of a currentframe (i.e., a frame header) being decoded. The current frame beingdecoded is a frame containing the current block.

The process 1000 can identify the parameterized motion model by decodingthe parameters of the parameterized motion model from the encodedbitstream. Decoding the parameters of the parameterized motion model caninclude decoding a motion model type from the encoded bitstream. Theprocess 1000 can then generate the parameters of the parameterizedmotion model corresponding to the motion model type. The process 1000can determine the parameters of the parameterized motion model asdescribed above with respect to FIG. 6.

At 1004, the process 1000 decodes the current block using theparameterized motion model in response to determining that the currentblock is encoded using the parameterized motion model. The current blockheader can include an indication identifying that the current block isencoded using the parameterized motion model. For example, the currentblock header can include an indicator of a global motion model type usedto encode the current block. For example, the indicator can indicatethat global motion was used to encode the current block or that noglobal motion was used to encode the current block (e.g., zero globalmotion).

In response to determining that the current block is encoded using theparameterized motion model, the process 1000 decodes the current blockusing the parameterized motion model. In response to determining thatthe current block is not encoded using the parameterized motion model,the process 1000 decodes the current block using translational motioncompensation.

The frame header for an inter-frame frame of the video sequence caninclude data indicating one or more parameterized motion models usableto encode or decode one or more blocks thereof. For example, the dataencoded to the frame header of an inter-frame frame can include theparameters of a parameterized motion model. The data may also include acoded flag indicating a number of parameterized motion models availableto the inter-frame frame.

In some implementations, a reference frame may not have a parameterizedmotion model. For example, there may be too many distinct motions withinthe reference frame to identify a global motion. In another example, theprediction errors determined for warped pixels based on motion modelsmay not satisfy the threshold. In such a case, blocks of frames usingthat reference frame can be encoded or decoded using zero motion. A zeromotion model may by default be encoded to the frame header of all orsome of the inter-frame frames of a video sequence.

In some implementations, a current block encoded using a parameterizedmotion model is decoded by warping the pixels of the encoded blockaccording to the parameterized motion model. The warped pixels of theencoded block are then interpolated. For example, the interpolation canbe performed using a 6-tap by 6-tap subpixel filter. In another example,the interpolation can be performed using bicubic interpolation. Bicubicinterpolation can include using a 4-tap by 4-tap window to interpolatethe subpixel values of an encoded block. Bicubic interpolation caninclude applying a horizontal sheer and a vertical sheer to an encodedblock.

FIG. 11 is an example 1100 of a global motion according toimplementations of this disclosure. The example 1100 includes a currentframe 1104 and a reference frame 1102. The current frame 1104 and thereference frame 1102 can be frames in a video sequence. The example 1100illustrates a situation where the camera, rather than objects beingcaptured by the camera, is moving. As indicated above, the cameramovement can include translations, rotations, zooms, shears, etc.

As also described above, a global motion (i.e., a parameterized motionmodel) for a reference frame of a current frame can be used to encode atleast a segment of the current frame. The global motion can be encoded,in an encoded bitstream, in the frame header of a current frame so thata decoder can use the global motion to decode at least a segment of thecurrent frame.

FIG. 12 is an example 1200 of encoding a current frame using globalmotion according to implementations of this disclosure. The example 1200includes a current frame 1202 to be encoded and reference frames REF_11204, REF_2 1206, and REF_3 1208. The current frame 1202 can be encodedusing any, or any combination, of the reference frames 1204, 1206,and/or 1208. The example 1200 includes optical flows 1210, 1216, 1222,global motions 1212, 1218, 1224, an aggregated residual 1230, andresiduals 1214, 1220, 1228. While not shown as such in FIG. 12, thecurrent frame 1202, the reference frames REF_1 1204, REF_2 1206, REF_31208, the optical flows 1210, 1216, 1222, and the global motions 1212,1218, 1224 have the same size. The aggregated residual 1230 and theresiduals 1214, 1220, 1228 illustrate partial residual blocks that areenlarged for clarity and visualization purposes. That is, the aggregatedresidual 1230 and the residuals 1214, 1220, 1228 illustrate residualsassociated with encoding a portion of the current frame 1202. As furtherdescribed below, the optical flows 1210, 1216, 1222 illustrate motion atthe block level; and the global motions 1212, 1218, 1224 illustratemovement induced by global motion.

In the video sequence that includes the current frame 1202 and thereference frames 1204, 1206, 1208, the foreground includes the EmpireState Building, and the background depicts other buildings. The videosequence illustrates an example of camera motion where a portion of theframes (e.g., the foreground including the Empire State Building) movesin one direction while the other portions of the frames (e.g., thebackground) move in another direction. The illustrated movement isslight and may not be easily perceptible. As such, the reference framescan be similar. As such, the global motion models and residuals are alsolikely to be similar. For simplicity, the example 1200 is described withrespect to one global motion model per reference frame. However, in thefollowing descriptions, multiple global motion models per referenceframe are understood to apply.

The optical flow 1210 describes the local motion between the currentframe and the reference frame REF_1 1204. That is, the optical flow 1210indicates, for each pixel of the current frame 1202, the movement of thepixel in the reference frame REF_1 1204. Given a pixel in the referenceframe REF_1 1204, the optical flow can indicate where the pixel moves inthe current frame. As such, for example, one pixel may be determined tomove three (3) pixels to the left, while another pixel may be determinedto move four (4) pixels to the right.

Similarly, the optical flow 1216 describes the local motion between thecurrent frame and the reference frame REF_2 1206, and the optical flow1222 describes the local motion between the current frame and thereference frame REF_3 1208. The optical flows 1210, 1216, 1222, asillustrated, indicate that these residuals are concentrated in theforeground of the current frame (i.e., the Empire State Building).

As described above, a global motion (also referred to as a parameterizedmotion model or a motion model) can have a small number of parametersand can induce a motion on every pixel of the reference frame. Forexample, assuming a translation to the right, then every pixel of thereference frame can be associated with moving to the right; and assuminga rotation, then every pixel moves a certain distance given by therotation.

The global motion 1212 illustrates the global motion associated with thereference frame REF_1 1204 with respect to the current frame 1202. Theglobal motion 1212 illustrates the movement of every pixel where themovement is induced by the global motion model associated with thereference frame REF_1 1204. While not important to describe the conceptsherein, the gray scales in the global motion 1212 (and the other globalmotion pictorial in the figures) are intended to depict an amount anddirection of global movement.

Similarly, the global motion 1218 illustrates the global motionassociated with the reference frame REF_2 1206 with respect to thecurrent frame 1202, and the global motion 1224 illustrates the globalmotion associated with the reference frame REF_3 1208 with respect tothe current frame 1202.

In the example 1200, the gray levels of the global motions 1212, 1218,1212 are, respectively, more similar to the gray levels of the portionsof the optical flows 1210, 1216, 1222 (e.g., the background) than theother portion (e.g., the Empire State Building). Similarly for theoptical flows 1210, 1216, 1222. That is, the example 1200 is intended todepict that the global motions describe more of the background of thecurrent frame 1202 than the foreground (i.e., the Empire StateBuilding). This is also illustrated using the residuals 1214, 1220,1228.

Considering the global motion from the reference frame REF_1 1204 to thecurrent frame 1202, the resulting residual 1214 illustrates that thedifferences (i.e., the residual) include foreground differences (i.e.,the Empire State Building). The global motion from the reference frameREF_1 1204 to the current frame 1202 can be one or more parameterizedmotion models that make the reference frame REF_1 1204 and the currentframe as similar as possible. Similarly, the residuals 1214, 1220illustrate that the motion models corresponding, respectively, to theglobal motions 1212, 1218 align more with the background than with theforeground of the current frame 1202. As described above, an error valueis typically associated with a residual. The error can be a mean squareerror between pixel values of the blocks of the current frame andprediction blocks. The error can be a sum of absolute differences error.The error can be a frequency-weighted error, such that high-frequencyerrors, which have higher coding costs, are weighted higher thanlow-frequency errors. Any other suitable error measure can be used. Inthe example 1200, the residuals 1214, 1220, and 1228 are calculated tohave respective mean square errors of 127.721, 312.328, and 358.795.

As described above, predicting the current frame 1202 can includepredicting each block of the current frame 1202 to determine theaggregated residual 1230. In a simple example, and ignoring local motion(e.g., inter-prediction), and assuming that each block of the currentframe 1202 is predicted using global motion, each of the blocks of thecurrent frame 1202 uses as its prediction block the block correspondingto the smallest residual in the reference frames 1204, 1206, 1208. Assuch, a first block of the current frame 1202 may be predicted from thereference frame REF_2 1206 along a line 1232, a second block may bepredicted from the reference frame REF_3 1208 along a line 1234, and athird block may be predicted from the reference frame REF_1 1204 along aline 1236. The aggregated residual 1230 has a corresponding calculatedmean square error of 118.387.

As described above and in the example 1200, the global motion models arecomputed independently. For example, first parameterized motion modelsthat best match first portions of a first reference frame to firstportions of a current frame are determined, second parameterized motionmodels that best match second portions of a first reference frame tosecond portions of a current frame are determined, etc. Calculating ordetermining parameterized motion models for a reference frame answersthe question: What is the best way of matching the reference frame tothe current frame? Given a set of reference frames, the calculating(e.g., identifying, etc.) of one or more parameterized motion models forone reference frame of the set of reference frames does not use resultsof the calculating of the parameterized motion models for any of theother reference frames.

In some implementations, and as further described below, global motion(i.e., parameterized motion models) of reference frames can be computedjointly. As mentioned above, the jointly computed global motion modelscan be known as diversified global motion.

For example, if a first global motion associated with a first referenceframe (e.g., the reference frame REF_1 1204) accurately (or withsufficient accuracy) describes a first portion (e.g., the background) ofa current frame (e.g., the current frame 1202), then determining theglobal motion associated with other reference frames (e.g., thereference frames REF_2 1206 and REF_3 1208) can ignore the first portionof the current frame and be focused on determining global motion modelsthat best describe the other portions (e.g., the foreground) of thecurrent frame. Similarly, the global motion associated with a secondreference frame (e.g., the reference frame REF_2 1206) may be a gooddescriptor of a second portion of the current frame (e.g., a firstportion of the foreground), and the global motion associated with athird reference frame (e.g., the reference frame REF_3 1208) may be agood descriptor of a third portion of the current frame (e.g., a secondportion of the foreground).

As such, with diversified global motion, and as further described withrespect to FIG. 13, given a set of reference frames, any one of thejointly determined global motions associated with the reference framesmay not be, by itself, a good overall descriptor of the global motion ofa current frame. However, collectively, the jointly determined globalmotion (i.e., the diversified motion) can better describe the globalmotions of the current frame because each of the jointly determinedglobal motions can be the best descriptor of a portion of the currentframe.

FIG. 13 is an example 1300 of diversified global motion according toimplementations of this disclosure. In the example 1300, the same orsimilar elements as those of FIG. 12 are indicated with the samenumerals. The example 1300 illustrates, for the current frame 1202,jointly determined global motions (i.e., diversified motion) associatedwith each of the reference frames REF_1 1204, REF_2 1206, REF_3 1208.

Global motion 1312 illustrates the global motion associated with thereference frame REF_1 1204. Global motion 1318 illustrates the globalmotion associated with the reference frame REF_2 1206. Global motion1324 illustrates the global motion associated with the reference frameREF_3 1208. For simplicity, only one motion model per reference frame isillustrated. However, the motion model associated with a frame can beone or more parameterized motion models, as described above. Forexample, the global motion associated with a reference frame can bemultiple parameterized motion models, each of which describes the motionin a group of blocks (e.g., a segment) of the current frame.

The global motions 1312, 1318, 1324 are jointly determined. Examples ofjointly determining the global motions are provided below. Optical flow1310 illustrates that the global motion 1312 is a good predictor of thebackground of the current frame 1202. By “good predictor,” it is meantthat the global motion 1312 describes the motion of the background ofthe current frame 1202 with sufficient accuracy. The Empire StateBuilding is barely perceptible in the optical flow 1310 and is morewashed into the background. Optical flow 1316 illustrates that theglobal motion 1318 is a good predictor of the foreground of the currentframe 1202. The Empire State Building is more perceptible in the opticalflow 1316 but is slightly washed into the background. Optical flow 1322illustrates that the global motion 1324 is a very good predictor of theforeground of the current frame 1202. The Empire State Building isclearly perceptible in the optical flow 1322.

Residual 1314 is the residual resulting from predicting all blocks ofthe current frame 1202 using the global motion 1312. The residual 1314has a mean square error (MSE) of 121.045. Residual 1320 is the residualresulting from predicting all blocks of the current frame 1202 using theglobal motion 1318. The residual 1320 has a mean square error (MSE) of565.076. Residual 1328 is the residual resulting from predicting allblocks of the current frame 1202 using the global motion 1324. Theresidual 1328 has a mean square error (MSE) of 636.596. Lighter areas ofthe residuals 1314, 1320, 1328 indicate higher residual values thanresiduals associated with darker areas of the residuals 1314, 1320,1328. The global motions 1318, 1324 generate high residuals for thebackground as illustrated, respectively, by the residuals 1320, 1328.

Aggregated residual 1330 is the residual that results from predictingthe current frame using the diversified global motion. That is, theaggregated residual 1330 is the residual that results from predictingthe current frame 1202 using the jointly determined global motions 1312,1318, 1324. Predicting the current frame 1202 jointly using the globalmotions 1312, 1318, 1324 means that, as explained above, for each blockof the current frame 1202, the best global motion 1312, 1318, 1324 (orother prediction) is selected to predict the block.

Using the diversified global motion to predict the current frame 1202results in a better prediction (MSE=59.309) than using global motionsthat are independently determined (MSE=118.387 of FIG. 12).

That is, when not considered jointly, the global motion 1318(MSE=565.076) associated with the reference frame REF_2 1206 performsworse than the global motion 1218 (MSE=312.328) of FIG. 12; and theglobal motion 1324 (MSE=636.596) associated with the reference frameREF_3 1286 performs worse than the global motion 1224 (MSE=358.795). Theglobal motions 1318, 1324 result in poor scores (i.e., high residuals)for the entirety of the current frame 1202, when considered separately.However, the global motions 1318, 1324 handle the foreground (e.g., theEmpire State Building) well, and thus improve compression. As such, whenpredicting the current frame 1202, the global motion 1312 can be used topredict blocks that are in the background, and one of the global motions1318, 1324 can be used to predict blocks that are in the foreground.

FIG. 14 is a flowchart diagram of a process 1400 for encoding a currentframe of video using diversified motion according to an implementationof this disclosure. Given a set of available reference frames forencoding the current frame, the process 1400 jointly determines, for atleast a subset of the available reference frames, respective motionmodels for encoding the current frame. The determined motion models areglobal motion models (i.e., parameterized motion models), as describedabove. The process 1400 then encodes blocks of the current frame usingthe jointly determined motion models.

The process 1400 can be implemented, for example, as a software programthat can be executed by a computing device, such as the transmittingstation 102. The software program can include machine-readableinstructions (e.g., executable instructions) that can be stored in amemory, such as the memory 204 or the secondary storage 214, and thatcan be executed by a processor, such as the CPU 202, to cause thecomputing device to perform the process 1400. The process 1400 can beimplemented in an encoder, such as the encoder 400 of FIG. 4. In atleast some implementations, the process 1400 can be performed in wholeor in part by the intra/inter prediction stage 402 of the encoder 400 ofFIG. 4.

The process 1400 can be implemented using specialized hardware orfirmware. Some computing devices can have multiple memories, multipleprocessors, or both. The steps or operations of the process 1400 can bedistributed using different processors, memories, or both. Use of theterms “processor” or “memory” in the singular encompasses computingdevices that have one processor or one memory as well as devices thathave multiple processors or multiple memories that can be used in theperformance of some or all of the recited steps or operations.

At 1402, the process 1400 jointly determines respective motion modelsfor reference frames that can be used for encoding the current frame. Asused herein, the term “determine” can mean create, construct, form,produce, generate, or determine in any manner whatsoever. The process1400 can determine respective motion models for all available referenceframes or for a subset of the available reference frames. For example,the process 1400 can determine respective motion models for three of theavailable reference frames. In an example, the three available referenceframes can be the golden frame, the alternative reference frame, and thelast reference frame. Examples of jointly determining respective motionmodels for reference frames are provided below with respect to FIGS. 15,16, and 18.

At 1404, the process encodes the current frame using the respectivemotion models. Encoding the current frame using the respective motionmodels can include encoding a current block of the current frame usingthe global motion model associated with the current block. Theassociated motion model can be as described with respect to FIG. 15, 17,or 18. In some implementations, a current block can be encoded using theassociated global motion model or a motion vector determined usingregular motion compensation. That is, a current block can be encodedusing the one of the associated motion model and the motion vectorresulting from regular motion compensation that results in the smallerresidual error. Encoding a current block can mean encoding the currentblock in an encoded bitstream, such as the compressed bitstream 420 ofFIG. 4.

In some implementations, the process 1400 can include additional stepsor operations. For example, the process 1400 can encode in an encodedbitstream (such as in the header of the current frame) the determinedmotion models. The process 1400 can encode the parameters of thedetermined motion models in the encoded bitstream. In an example, and asdescribed with respect to FIG. 16, for each of three reference frames,four different candidate motion models can be computed. One of the fourcomputed candidate motion models per reference frame may be selected forthe encoding of the current frame. As such, the parameters of theselected candidate motion models can be encoded in the compressedbitstream. The selected candidate motion models can be such that theyjointly cover the current frame. That is, each block of the currentframe can be associated with one of the candidate motion models.

In some implementations, other steps or operations not presented anddescribed herein may be used in the process 1400. Furthermore, not allillustrated steps or operations may be required to implement a techniquein accordance with the disclosed subject matter.

FIG. 15 is a flowchart diagram of a process 1500 for jointly determiningmotion models according to an implementation of this disclosure. Theprocess 1500 can be implemented at 1402 of the process 1400.

The process 1500 can generate several candidate motion models for eachof the reference frames. The process 1500 then selects the bestcombination of motion models. The best combination is the combination ofmotion models that results in the smallest aggregated residual for thecurrent frame. In an example, a combination of motion models includesone candidate motion model for each of the reference frames. In anotherexample, a combination of motion models includes zero or more candidatemotion models for each of the reference frames. Each of the combinationsof motion models corresponds to a diversified motion model. The bestcombination of motion models is a diversified motion model that isselected for encoding the current frame.

For simplicity of explanation, the process 1500 is described withrespect to three reference frames and four candidate motion models perreference frame. However, more or fewer reference frames can be used,and more or fewer candidate motion models can be generated (e.g.,identified, computed, etc.) for each reference frame. The same or adifferent number of candidate motion models can be generated for eachreference frame. The candidate motion models are not necessarily optimalmotion models. That is, the candidate motion models do not necessarilyhave to result in the smallest MSEs for the current frame as a whole.

The process 1500 can be described as a two-stage process: In the firststage, several (e.g., four) candidate motion models for each currentframe/reference frame pair are independently generated; in the secondstage, a joint optimization over all the reference frames is performedto select one collective motion (i.e., a combination of motion models)to be used to encode the current frame. In an example, the collectivemotion used to encode the current frame includes, for each referenceframe, one global motion model selected from four candidate motionmodels for the reference frame.

At 1502, the process 1500 determines respective aggregated residuals forcombinations of candidate motion models.

In an example, each of the combinations of candidate motion modelsconsists of one respective candidate motion model for each of thereference frames. In another example, each of the combinations ofcandidate motion models consists of zero or more respective candidatemotion models for each of the reference frames. More generally, acombination of candidate motion models can include any number ofcandidate motion models and the number of candidate motion models is notlimited to the number of reference frames.

Determining respective aggregated residuals for combinations ofcandidate motion models can include generating respective candidatemotion models for a reference frame independently of the candidatemotion models of other reference frames.

In some implementations of the process 1500, combinations of candidatemotion models that do not include one respective candidate motion modelfor each of the reference frames can be determined at 1502. For example,and using the examples of FIG. 16, combinations that include two,instead of three, candidate motion models can be determined.

FIG. 16 is an example 1600 of combinations of motion models according toimplementations of this disclosure. In the example 1600, four candidatemotion models are generated for the reference frame REF_1 1204, namely,the candidate motion models 1602-1608; four candidate motion models aregenerated for the reference frame REF_2 1206, namely, the candidatemotion models 1612-1618; and four candidate motion models are generatedfor the reference frame REF_3 1208, namely, the candidate motion models1622-1628.

The candidate motion models can be generated in any number of ways. Forexample, the candidate motion models for a reference frame can eachcorrespond to a motion model type (e.g., the translational motion modeltype, the similarity motion model type, the affine motion model type,and the homographic motion model type). For example, each of thecandidate motion models can be generated using a different erroradvantage. For example, the parameterized motion models associated withdifferent segments of the current frame can be used as the candidatemotion models for the reference frame. For example, the current framecan be partitioned into segments, and a parameterized motion model canbe determined for each segment. The number of segments can correspond tothe number of candidate motion models to be generated. Blocks of thecurrent frame can be assigned to the segments in any way. In an example,a first group of contiguous blocks (e.g., 25% of the blocks) areassigned to a first group, the next group of contiguous blocks areassigned to a second group, and so on.

In an implementation, determining respective aggregated residuals forcombinations of candidate motion models can include generating arespective residual for some of the candidate motion models of areference frame. For example, for each candidate motion model of thegenerated candidate motion models, the process 1500 can generate theresidual that corresponds to predicting the current frame using thecandidate motion model and the corresponding reference frame.

FIG. 16 illustrates respective residuals. Residuals 1603-1609 are theresiduals that result from predicting the current frame 1202 using,respectively, the candidate motion models 1602-1608. Residuals 1613-1619are the residuals that result from predicting the current frame 1202using, respectively, the candidate motion models 1612-1618. Residuals1623-1629 are the residuals that result from predicting the currentframe 1202 using, respectively, the candidate motion models 1622-1628.Predicting the current frame using a candidate motion model can includepartitioning the current frame into fixed blocks of size N×M andpredicting each of the blocks using the candidate motion model. N and Mcan be integer values selected from a set that includes the values 4, 8,16, 32, 64, and 128.

The process 1500 can perform an exhaustive search, jointly, over thereference frames (e.g., the three reference frames REF_1, REF_2, andREF_3) to determine which combination of candidate motion models fromthe candidate motion models minimizes the reconstruction error of thecurrent frame. In FIG. 16, as four candidate motion models are generatedper reference frame, the process 1500 can determine an aggregatedresidual for each of the 64 (i.e., 4*4*4=64) combinations of candidatemotion models. Example combinations of candidate motion models include:candidate motion models 1602, 1612, 1622; candidate motion models 1602,1612, 1624; candidate motion models 1602, 1612, 1626; candidate motionmodels 1602, 1612, 1628; candidate motion models 1604, 1612, 1622;candidate motion models 1604, 1612, 1624; candidate motion models 1604,1612, 1626; candidate motion models 1604, 1612, 1628; and so on.

As mentioned above, in an example, a combination of candidate motionmodels can include zero or more candidate motion models per referenceframe. As such, and assuming that each combination of candidate motionmodels includes three candidate motion models, then 220 combinations ofcandidate motion models are possible. The 220 possible combinationscorrespond to selecting any three candidate motion models of the totalof 12 (i.e., candidate motion models 1602-1608, 1612-1618, and1622-1628) available candidate motion models. Example combinations ofcandidate motion models include: candidate motion models 1602, 1604,1606; candidate motion models 1602, 1604, 1622; candidate motion models1606, 1616, 1628; and so on. In an example where each combination ofcandidate motion models includes four candidate motion models, examplesof combinations of candidate motion models include: candidate motionmodels 1602, 1604, 1606, 1612; candidate motion models 1602, 1604, 1606,1608; candidate motion models 1602, 1604, 1612, 1628; and so on.

Whereas this disclosure may refer to selecting one respective candidatemotion model for each of the reference frames for simplicity ofexplanation, it is to be understood, as described above, that acombination of candidate motion models can include zero or morecandidate motion models for each reference frame and/or that the numberof motion models in combinations of motion models is not limited to thenumber of reference frames.

The number of motion models per combination can depend on apredetermined desired number of motion models per combination. Forexample, the predetermined desired number can be an input or aconfiguration. In another example, a process for encoding a currentframe of video using diversified motion, such as the process 1400, canuse a range of desired number of motion models. For example, given arange [3-5], the process can generate combinations of 3 motion models,combinations of 4 motion models, and combinations of 5 motion models.The best combination of motion model, as described herein, can be usedto encode a frame of video. The number of combinations of motion modelscan also be an input, a configuration, or the like.

In an implementation, determining respective aggregated residuals forcombinations of candidate motion models can include, for a currentblock, determining a first residual block using the first candidatemotion model, determining a second residual block using the secondcandidate motion model, determining a third residual block using thethird candidate motion model, and adding, to an aggregated residual ofthe current frame, the one of the first residual block, the secondresidual block, and the third residual block corresponding to a smallestresidual block.

For example, for the combination that includes candidate motion models1608, 1616, 1624, for each block of size 8×8 in the current frame, theprocess 1500 can select the one of (1608, reference frame REF_1 1204),(1616, reference frame REF_2 1206), (1624, reference frame REF_3 1208)pair that provides the best prediction (i.e., the smallest residual).The smallest residual for each block is added to the aggregated residualof the combination of the candidate motion models. Residual 1630 is anexample of an aggregated residual of a combination of candidate motionmodels.

As described above, the process 1500 can associate a respectiveaggregated residual with each of the combinations of candidate motionmodels. At 1504, the process 1500 can select the combination ofcandidate motion models that corresponds to the smallest aggregatedresidual. As such, each of the blocks of the current frame can beassociated with a global motion model that corresponds to the smallestaggregated error for the current frame.

The diversified motion described herein does not assume or rely on thefact that the reference frames are similar to each other. Thediversified motion according to implementations of this disclosure canbe used with any set of reference frames.

In some implementations, one or more of the reference frames used fordetermining a diversified motion model for a current frame may not bedisplayable frames of the video sequence that includes the currentframe.

For example, if a video sequence includes an object of interest (e.g., afamous personality, a landmark, etc.), then frames or images of theobject of interest may be included in the video sequence as referenceimages. These reference images are non-displayable frames because theyare not part of the video sequence per se. However, these referenceimages may be used as reference frames to provide better prediction forthose frames (or portions of frames) of the video sequence that includethe object of interest. Such reference frames may be used for predictingat least aspects of the object of interest. For example, such referenceframes may be used to better predict the hairstyle (e.g., an aspect) ofa person (e.g., the object of interest).

In some implementations, one or more of the reference frames used fordetermining a diversified motion model for a current frame may be aframe temporally distant from the current frame. Typically, referenceframes used for predicting a current frame are frames that aretemporally proximate to the current frame. However, a reference frameused to determine a diversified motion model can be a temporally distantframe.

For example, assume a panoramic video sequence of a landscape is taken.The panoramic video sequence includes a first frame that includes anaspect of the landscape (e.g., a rock formation). A second frame that istaken 10 minutes later and a third frame that is taken 12 minutes lateralso include the same aspect. In typical prediction techniques, thefirst frame would not be used as a reference frame for predicting thesecond and/or third frames. However, using diversified motion modelsaccording to implementations of this disclosure, the first frame can beused as a reference frame for predicting the aspect of the landscape inthe second and third frames, even though the first frame is temporallydistant from the second and third frames. The first frame can be tagged(e.g., identified, selected, and the like) as a reference frame forpredicting frames that include the aspect.

In the process 1500 of jointly determining motion models, the candidatemotion models for a reference frame are selected independently of thecandidate motion models of other reference frames. In other examples,and as further described with respect to FIGS. 17-18, jointlydetermining motion models can generate one or more candidate motionmodels of a reference frame using information available about thecandidate motion models of the other reference frames. That is, allreference frames (e.g., candidate motion models of all the referenceframes) can be used simultaneously to determine the diversified motionmodel.

FIG. 17 is a flowchart diagram of a process 1700 for jointly determiningmotion models according to another implementation of this disclosure.The process 1700 is described with respect to three reference frames.However, any number of reference frames can be available.

In an example, for each of the reference frames, the process 1700assigns a respective initial motion model. In another example, areference frame can be assigned zero or more initial motion models. Theprocess 1700 then iteratively refines one of the motion models by fixingthe other two motion models. The process 1700 iterates until anobjective function is met. Depending on the semantics of the objectivefunction, “until the objective function is met” can mean until theobjective function is met, while the objective function is met, untilthe objective function is not met, while the objective function is notmet, or other semantics. The objective function can be any one or moreconditions, processing, evaluation, and the like that cause the process1700 to proceed from 1706 to 1726. At the end of each iteration (andbefore the first iteration), a current diversified motion model isdetermined. As such, the current diversified motion model associates aglobal motion model with each block of the current frame.

At 1702, the process 1700 assigns respective initial motion models tothe motion models of the reference frames. Given the reference framesREF_1, REF_2, and REF_3, the process 1700 can assign, respectively,initial motion models M1, M2, and M3. As mentioned above, a referenceframe can be assigned zero or more initial motion models. For example,M1 and M2 may correspond to REF_1 and M3 may correspond to REF_3. Assuch, it is to be understood that the number of initial motion models isnot limited by the number of reference frames. For example, the initialmotion models may be M1, M2, M3, and M4 where M1 and M3 correspond toREF_1, M2 corresponds to REF_2, and M4 corresponds to REF_3.

The initial motion models can be any motion models generated asdescribed above. The initial motion models do not necessarily have to bethe most optimal motion models associated with the correspondingreference frame. For example, the initial motion models can be generatedusing a least complex model type. For example, the initial motion modelscan be generated ignoring the error advantages associated with theinitial motion models. The combination of motion models M1, M2, M3constitutes a current diversified motion model.

At 1704, the process 1700 determines a cost of encoding the currentframe using the motion models (i.e., using the current diversifiedmotion model). The process 1700 determines the encoding cost for eachblock of the current frame as the minimum cost among following M1 inREF_1, following M2 in REF_2, and following M3 in REF_3. For example,the process 1700 can determine the residual value (MSE) associated withencoding the blocks of the current frame using the initial motion models(M1, M2, M3).

At 1706, the process 1700 determines whether an objective function ismet. If so, the process 1700 ends at 1726. Otherwise, the process 1700proceeds to 1708 to perform an iteration for improving the currentmotion models (i.e., the current diversified motion model). That is, theprocess 1700 iteratively refines (by performing 1708-1724), until theobjective function is met, the motion models M1, M2, M3 in order toreduce the cost of encoding the current frame using the currentdiversified motion model. When the objective function is met, thecurrent diversified motion model associates, with each block of thecurrent frame, a global motion model, such that the blocks of thecurrent frame can be encoded (such as at 1404 of FIG. 14) using theassociated motion models. The objective function is further explainedbelow.

The motion models can be refined in any number of ways. For example,refining a motion model can mean generating another parameterized motionmodel, as described above, for the same blocks that use the motionmodel. The another parameterized motion model can, for example, use amotion model type different than that of the motion model. For example,refining a motion model can mean generating a motion model for a subsetof the blocks of the current frame. For example, the subset can includeall the blocks except for one, two, or any number of blocks. In anotherexample, RANSAC over all the blocks of the current frame, weighted bythe respective encoding costs of the blocks, can be used. That is,RANSAC can be biased toward those blocks of the current frame for whichthe current diversified motion model is not optimal. That is, RANSAC canbe biased toward those blocks associated with the highest errors.

At 1708, the process fixes the motion models M2 and M3. That is, theprocess 1700 uses, without varying, the last determined motion models M2and M3. As such, in the first iteration, the initial motion modelsassigned to the motion models M2 and M3, at 1702, are used.

At 1710, the process 1700 generates candidate motion models M′₁ forREF_1. In an example, four candidate motion models M′₁ can be generated(e.g., M′_(1,1), M′_(1,2), M′_(1,3), M′_(1,4)). However, any number ofcandidate motion models can be generated.

For each of the candidate motion models, the process 1700 determines(not shown in FIG. 17) a respective error of encoding the current frameusing M2, M3, and the motion model candidate. In an example, therespective errors can be determined similarly to 1502 of FIG. 15. Forexample, aggregated residuals for the combinations (M′_(1,1), M2, M3),(M′_(1,2), M2, M3), (M′_(1,3), M2, M3), and (M′_(1,4), M2, M3) aredetermined.

At 1712, the process 1700 sets M1 to the best candidate motion modelsM′₁ that result in the smallest aggregated residual, similarly to 1504of FIG. 15. Assuming, for example, that M′_(1,2) is selected, then thenew combination M1 (set to M′_(1,2)), M2, M3 constitutes the currentdiversified motion model.

At 1714-1718, the process 1700 performs actions similar to thosedescribed with respect to 1708-1712, except that M1 and M3 are fixed at1714, candidate motion models M′₂ (e.g., M′_(2,1), M′_(2,2), M′_(2,3),M′_(2,4)) are generated for M2 at 1716, and M2 is set to the bestcandidate motion model M′₂. M1 is fixed to the candidate motion modelM′₁ of 1712.

At 1720-1724, the process 1700 performs actions similar to thosedescribed with respect to 1708-1712 and 1714-1718, except that M1 and M2are fixed at 1720, candidate motion models M′₃ (e.g., M′_(3,1),M′_(3,2), M′_(3,3), M′_(3,4)) are generated for M3 at 1722, and M3 isset to the best candidate motion model M′₃. M1 is fixed to the candidatemotion model M′₁ of 1712. M2 is fixed to the candidate motion model M′₂of 1718.

To summarize the operations 1708-1712, 1714-1718, and 1720-1724, theprocess 1700 generates first motion model candidates (e.g., M′_(2,1),M′_(2,2), M′_(2,3), M′_(2,4)) for a first reference frame (e.g., REF_2),and determines, for each of the first motion model candidates (e.g.,M′_(2,1), M′_(2,2), M′_(2,3), M′_(2,4)), a respective error of encodingthe current frame using the respective motion models (e.g., M1, M3) ofthe other frames (e.g., REF_1, REF_3) and the motion model candidate.The process 1700 then sets the motion model (e.g., M2) of (i.e.,associated with) the first reference frame (e.g., REF_2) to one of thefirst motion model candidates (e.g., M′_(2,1), M′_(2,2), M′_(2,3),M′_(2,4)) corresponding to a smallest aggregated residual.

The process 1700 then proceeds back to 1706 to determine whether theobjective function is met. In an example, the objective function can bea number of iterations. The number of iterations can be any number(e.g., 2, 3, 4, etc.). As such, the process 1700 ends at 1726 when theprocess 1700 performs 1708-1724 the number of iterations times.

In another example, the objective function can be related to an error ofencoding the current frame using the motion models. The error thresholdcan relate to the error associated with encoding the current frame usingthe motion models M1, M2, and M3 set during an iteration of 1708-1724.That is, the error threshold can relate to the error associated withencoding the current frame using the current diversified motion model.The error can be a mean square error between pixel values of a block ofthe current and pixel values of a prediction block of a reference frame.The error can be a sum of absolute differences error. Any other suitableerror measure can be used.

In an example, the objective function can be such that the error must beimproved by a certain error threshold from one iteration to the next. Assuch, if an iteration does not improve the error by at least the errorthreshold, then the process 1700 proceeds from 1706 to 1726. Forexample, the objective function can be considered met when an iterationdoes not improve the encoding of a given percentage of the blocks of thecurrent frame. Said another way, the objective function is such that,with every iteration, coding of the given percentage of the blocks ofthe current frame must be improved. In an example, the percentage can bea third of the blocks of the frame. In another example, 10% of theblocks must be improved. Any percent threshold can be used.

In some situations, it may not be possible to identify (i.e., determine,generate, etc.), within a reasonable number of iterations or amount ofcomputing resources, an optimal motion model for some pixels or blocksof the current frame. As such, errors associated with these blocks orpixels are excluded from the error calculation, as these blocks do notprovide a measure of the iteration-to-iteration improvements.Accordingly, a robustness measure can be added to the objective functionby adding condition(s) that exclude these blocks from the error (e.g.,the aggregated error) determination. Excluding some blocks from theaggregated error calculations may result in a high cost (i.e., asub-optimal diversified motion model) of encoding such blocks. However,excluding such blocks prevents a situation where such blocks can skew orbias an otherwise optimal diversified motion model for the rest of theblocks of the current frame.

In an example, blocks of the current frame that do not meet a block-wiseerror threshold are excluded from the determining of the error. In anexample, the block-wise error threshold can be “in the 95^(th)percentile of the errors.” That is, the blocks of the current frame thatare associated with the 5% highest errors are excluded from thedetermination of the error to be used in the objective function.

In another example, block errors that exceed a threshold can be excludedfrom the determining of the error. For example, and assuming that thethreshold is 200, then if the error (e.g., MSE) associated withpredicting a block of the current frame is greater than 200, then theerror is not added to the aggregated residual.

FIG. 18 is a flowchart diagram of a process 1800 for jointly determiningmotion models according to yet another implementation of thisdisclosure. The process 1800 is described with respect to threereference frames. However, any number of reference frames can beavailable.

The process 1800 simultaneously updates the motion models M1, M2, and M3and the blocks of the current frame to which the motion models M1, M2,and M3 are assigned. As such, the process 1800 optimizes two dependentquantities: the motion models (which are determined based on the blocksassigned to the motion models) and the block assignments (i.e., whichblocks are assigned to which motion models). As used herein, “a block ofthe current frame is assigned to a motion model” means that the motionmodel can be used to predict the block. As used herein, “a motion modelis assigned to a block of the current frame” means that the motion modelcan be used to predict the block.

At 1802, the process 1800 makes an initial assignment of initial motionmodels to blocks of the current frame. In an example, the process 1800can partition the current frame into a number of groups of blocks thatis equal to the number of reference frames. Each group of blocksconstitutes a subset of blocks of the current frame.

An initial motion model can be generated for each of the groups withrespect to a respective reference frame. For example, given threereference frames REF_1, REF_2, and REF_3 and three groups G1, G2, andG3, the process 1800 can generate a motion model M1 for predicting theblocks of the group G1 from the reference frame REF_1, a motion model M2for predicting the blocks of the group G2 from the reference frameREF_2, and a motion model M3 for predicting the blocks of the group G3from the reference frame REF_3. The motion models M1, M2, and M3constitute the respective motion models of the reference frames andconstitute the current diversified motion model.

The current frame can be partitioned into groups in any number of ways.For example, each block of the current frame can be randomly assigned toa group. In another example, a consecutive (e.g., in a raster scanorder) percentage of the blocks can be assigned to each group. Forexample, the first third of the blocks can be assigned to the group G1,the second third of the blocks can be assigned to the group G2, and thethird third of the blocks can be assigned to the group G3. Other ways ofpartitioning the current frame are possible. The segments can includethe same or a different number of blocks.

At 1804, the process 1800 determines whether an objective function ismet. If so, the process 1800 ends at 1810. Otherwise, the process 1800proceeds to 1806 to perform an iteration for improving the currentmotion models and the block assignments. The objective function can beas described with respect to 1706 of FIG. 17.

At 1806, the process 1800 refines the respective motion models of thereference frames assigned to respective groups of blocks of the currentframe. As such, the process 1800 updates (i.e., refines) the motionmodels M1, M2, and M3 with the block assignments fixed. The motionmodels can be refined as described with respect to FIG. 17.

The motion models M1, M2, and M3 are refined using the same groups ofblocks (e.g., G1, G2, and G3) assigned to each of the motion models.That is, during the refinement operation, each motion model is refinedwhile considering only the blocks assigned to the motion model in orderto reduce the overall encoding cost, using the motion model, of theblocks assigned to the motion model.

As such, refining the respective motion models of the reference framesassigned to the respective groups of blocks of the current frame caninclude assigning a first motion model of a first reference frame of thereference frames to a first group of blocks of the current frame,assigning a second motion model of a second reference frame of thereference frames to a second group of blocks of the current frame, andupdating the first motion model and the second motion model to minimizea cost of encoding the first group of blocks and the second group ofblocks.

At 1808, the process 1800 updates an assignment of the respective motionmodels to blocks of the current frame. That is, the block assignment ofall the blocks of the current frame may be updated with all the motionmodels (i.e., the refined motion models M1, M2, and M3 of 1806) fixed.The refined motion models M1, M2, M3 correspond to the currentdiversified motion model. The process 1800 can update the blockassignment by picking (i.e., associating, etc.), for each block of thecurrent block, the one of the refined motion models M1, M2, and M3 thatminimizes the block encoding cost. As such, the block 1808 can result indifferent blocks being included in each of the groups G1, G2, and G3.The updated groups G1, G2, and G3 are used in a next iteration at block1806. As such, a block that is included in a group of the groups G1-G3can be said to be associated with the motion model of the group. It isto be noted that, at 1808, the updating of the motion models to blocksmay result in some of the motion models not being assigned to any blocksof the current frame. Accordingly, when the process 1800 reaches 1810,some of the motion models may not be assigned to any of the blocks ofthe current frame.

As such, updating the assignment of the respective motion models toblocks of the current frame can include reassigning the first motionmodel and the second motion model to blocks of the first group of blocksand the second group of blocks to minimize a cost of encoding the blocksof the first group of blocks and the second group of blocks.

FIG. 19 is a flowchart diagram of a process 1900 for decoding a currentframe according to one implementation of this disclosure. The process1900 receives an encoded bitstream, such as the compressed bitstream 420of FIG. 5. The process 1900 may be performed by a decoder. For example,the process 1900 can be performed in whole or in part by theintra/inter-prediction stage 508 of the decoder 500. The process 1900can be performed in whole or in part during the reconstruction path(shown by the dotted connection lines) of the encoder 400 of FIG. 4.Implementations of the process 1900 can be performed by storinginstructions in a memory, such as the memory 204 of the receivingstation 106 or the transmitting station 102, to be executed by aprocessor, such as the CPU 202, for example.

The process 1900 can be implemented using specialized hardware orfirmware. Some computing devices can have multiple memories, multipleprocessors, or both. The steps or operations of the process 1900 can bedistributed using different processors, memories, or both. Forsimplicity of explanation, the process 1900 is depicted and described asa series of steps or operations. However, the teachings in accordancewith this disclosure can occur in various orders and/or concurrently.Additionally, steps or operations in accordance with this disclosure mayoccur with other steps or operations not presented and described herein.Furthermore, not all illustrated steps or operations may be used toimplement a method in accordance with the disclosed subject matter.

At 1902, the process 1900 decodes, from an encoded bitstream, respectivemotion models for reference frames of the current frame. The respectivemotion models for the reference frames constitute a diversified motionmodel for predicting the current frame. The motion models of thereference frames were encoded in the bitstream by an encoder, such asthe encoder 400 of FIG. 4, where the encoder jointly determined therespective motion models. The encoder may have jointly determined therespective motion models using one of the processes 1500, 1700, or 1800.

At 1904, the process 1900, in response to determining that a currentblock of the current frame is encoded using at least one of therespective motion models, decodes the current block using the at leastone of the respective motion models.

For simplicity of explanation, the processes 600, 1000, 1400, 1500,1700, 1800, and 1900 are depicted and described as a series of steps oroperations. However, the steps or operations in accordance with thisdisclosure can occur in various orders and/or concurrently.Additionally, other steps or operations not presented and describedherein may be used. Furthermore, not all illustrated steps or operationsmay be required to implement a method in accordance with the disclosedsubject matter.

The aspects of encoding and decoding described above illustrate someexamples of encoding and decoding techniques. However, it is to beunderstood that encoding and decoding, as those terms are used in theclaims, could mean compression, decompression, transformation, or anyother processing or change of data.

The word “example” is used herein to mean serving as an example,instance, or illustration. Any aspect or design described herein as“example” is not necessarily to be construed as being preferred oradvantageous over other aspects or designs. Rather, use of the word“example” is intended to present concepts in a concrete fashion. As usedin this application, the term “or” is intended to mean an inclusive “or”rather than an exclusive “or.” That is, unless specified otherwise orclearly indicated otherwise by the context, the statement “X includes Aor B” is intended to mean any of the natural inclusive permutationsthereof. That is, if X includes A; X includes B; or X includes both Aand B, then “X includes A or B” is satisfied under any of the foregoinginstances. In addition, the articles “a” and “an” as used in thisapplication and the appended claims should generally be construed tomean “one or more,” unless specified otherwise or clearly indicated bythe context to be directed to a singular form. Moreover, use of the term“an implementation” or the term “one implementation” throughout thisdisclosure is not intended to mean the same embodiment or implementationunless described as such.

Implementations of the transmitting station 102 and/or the receivingstation 106 (and the algorithms, methods, instructions, etc., storedthereon and/or executed thereby, including by the encoder 400 and thedecoder 500) can be realized in hardware, software, or any combinationthereof. The hardware can include, for example, computers, intellectualproperty (IP) cores, application-specific integrated circuits (ASICs),programmable logic arrays, optical processors, programmable logiccontrollers, microcode, microcontrollers, servers, microprocessors,digital signal processors, or any other suitable circuit. In the claims,the term “processor” should be understood as encompassing any of theforegoing hardware, either singly or in combination. The terms “signal”and “data” are used interchangeably. Further, portions of thetransmitting station 102 and the receiving station 106 do notnecessarily have to be implemented in the same manner.

Further, in one aspect, for example, the transmitting station 102 or thereceiving station 106 can be implemented using a general-purposecomputer or general-purpose processor with a computer program that, whenexecuted, carries out any of the respective methods, algorithms, and/orinstructions described herein. In addition, or alternatively, forexample, a special-purpose computer/processor, which can contain otherhardware for carrying out any of the methods, algorithms, orinstructions described herein, can be utilized.

The transmitting station 102 and the receiving station 106 can, forexample, be implemented on computers in a video conferencing system.Alternatively, the transmitting station 102 can be implemented on aserver, and the receiving station 106 can be implemented on a deviceseparate from the server, such as a handheld communications device. Inthis instance, the transmitting station 102, using an encoder 400, canencode content into an encoded video signal and transmit the encodedvideo signal to the communications device. In turn, the communicationsdevice can then decode the encoded video signal using a decoder 500.Alternatively, the communications device can decode content storedlocally on the communications device, for example, content that was nottransmitted by the transmitting station 102. Other suitable transmittingand receiving implementation schemes are available. For example, thereceiving station 106 can be a generally stationary personal computerrather than a portable communications device, and/or a device includingan encoder 400 may also include a decoder 500.

Further, all or a portion of implementations of the present disclosurecan take the form of a computer program product accessible from, forexample, a computer-usable or computer-readable medium. Acomputer-usable or computer-readable medium can be any device that can,for example, tangibly contain, store, communicate, or transport theprogram for use by or in connection with any processor. The medium canbe, for example, an electronic, magnetic, optical, electromagnetic, orsemiconductor device. Other suitable mediums are also available.

The above-described embodiments, implementations, and aspects have beendescribed in order to facilitate easy understanding of this disclosureand do not limit this disclosure. On the contrary, this disclosure isintended to cover various modifications and equivalent arrangementsincluded within the scope of the appended claims, which scope is to beaccorded the broadest interpretation as is permitted under the law so asto encompass all such modifications and equivalent arrangements.

What is claimed is:
 1. A method for encoding a current frame of a video,comprising: jointly determining respective motion models for referenceframes, the reference frames used for encoding the current frame, by:determining respective aggregated residuals for combinations ofcandidate motion models; and selecting a combination of candidate motionmodels corresponding to a smallest aggregated residual, wherein therespective motion models correspond to the candidate motion models ofthe selected combination; and encoding the current frame using therespective motion models.
 2. The method of claim 1, wherein determiningrespective aggregated residuals for combinations of candidate motionmodels comprises: generating a respective residual for some of thecandidate motion models of a reference frame.
 3. The method of claim 2,wherein a combination of the combinations of candidate motion modelscomprises a first candidate motion model for a first reference frame, asecond candidate motion model for a second reference frame, and a thirdcandidate motion model for a third reference frame, and whereindetermining respective aggregated residuals for combinations ofcandidate motion models comprises: determining, for a current block, afirst residual block using the first candidate motion model;determining, for the current block, a second residual block using thesecond candidate motion model; determining, for the current block, athird residual block using the third candidate motion model; and adding,to an aggregated residual of the current frame, the one of the firstresidual block, the second residual block, and the third residual blockcorresponding to a smallest residual block.
 4. The method of claim 2,wherein generating the respective residual for some of the candidatemotion models of the reference frame comprises: partitioning the currentframe into blocks; and predicting the blocks using one of the candidatemotion models of the reference frame.
 5. The method of claim 1, whereinfour candidate motion models are used for each of the reference framesof the current frame.
 6. The method of claim 1, wherein the referenceframes of the current frame comprise a reference image that is anon-displayable frame of the video.
 7. The method of claim 1, whereindetermining respective aggregated residuals for combinations ofcandidate motion models comprises: generating respective candidatemotion models for a reference frame independently of the candidatemotion models of other reference frames.
 8. The method of claim 1,wherein the current frame comprises a first group of blocks and a secondgroup of blocks, and wherein determining respective aggregated residualsfor combinations of candidate motion models comprises: generating acandidate motion model for the first group of blocks; and determining anaggregated residual for the current frame using the candidate motionmodel for the first group of blocks and using other motion models forthe second group of blocks.
 9. An apparatus for encoding a current frameof a video, the apparatus comprising: a processor configured to executeinstructions stored in a non-transitory storage medium to: jointlydetermine motion models for reference frames; and encode the currentframe using the motion models.
 10. The apparatus of claim 9, wherein arespective motion model is determined for a respective reference frame,and wherein to jointly determine the motion models for the referenceframes comprises to: assign respective initial motion models to themotion models; determine a cost of encoding the current frame using themotion models; and iteratively refine, until an objective function ismet, the motion models to reduce the cost of encoding the current frameusing the respective initial motion models.
 11. The apparatus of claim10, wherein the reference frames consist of a first reference frame andother reference frames, and wherein to iteratively refine, until theobjective function is met, the motion models to reduce the cost ofencoding the current frame using the respective initial motion modelscomprises to: generate motion model candidates for the first referenceframe; determine, for each motion model candidate of the motion modelcandidates, a respective error of encoding the current frame using therespective motion models of the other frames and the motion modelcandidate; and set the motion model of the first reference frame to oneof the first motion model candidates corresponding to a smallestaggregated residual.
 12. The apparatus of claim 11, wherein theobjective function is related to an error of encoding the current frameusing the motion models.
 13. The apparatus of claim 12, wherein theerror of encoding the current frame using the motion models is a meansquare error.
 14. The apparatus of claim 12, wherein to determine therespective error of encoding the current frame using the respectivemotion models comprises to: exclude, from determining of the respectiveerror, blocks of the current frame that meet a block-wise errorthreshold.
 15. The apparatus of claim 14, wherein to determine therespective error of encoding the current frame using the respectivemotion models comprises to: exclude, from the determining of therespective error, block errors that exceed a threshold.
 16. Theapparatus of claim 10, wherein the objective function is a number ofiterations.
 17. The apparatus of claim 10, wherein to jointly determinethe motion models for the reference frames comprises to: until anobjective function is met: refine respective motion models of thereference frames assigned to respective groups of blocks of the currentframe; and update an assignment of the respective motion models toblocks of the current frame.
 18. The apparatus of claim 17, wherein torefine the respective motion models of the reference frames assigned tothe respective groups of blocks of the current frame comprises to:assign a first motion model of a first reference frame of the referenceframes to a first group of blocks of the current frame; assign a secondmotion model of a second reference frame of the reference frames to asecond group of blocks of the current frame; and update the first motionmodel and the second motion model to minimize a cost of encoding theblocks of the first group of blocks and the second group of blocks. 19.The apparatus of claim 18, wherein to update the assignment of therespective motion models to blocks of the current frame comprises to:reassign the first motion model and the second motion model to blocks ofthe first group of blocks and the second group of blocks to minimize acost of encoding the blocks of the first group of blocks and the secondgroup of blocks.
 20. A method for decoding a current frame, comprising:decoding, from an encoded bitstream, respective motion models forreference frames of the current frame, wherein an encoder jointlydetermined the respective motion models; and in response to determiningthat a current block of the current frame is encoded using at least oneof the respective motion models, decoding the current block using the atleast one of the respective motion models.